Unplanned perioperative hypothermia is a common surgical risk. Unplanned hypothermia is defined as a body temperature below 36° C (96.8° F) during any phase of the perioperative period. Perioperative nurses at a Midwestern tertiary pediatric hospital developed an evidence-based clinical practice guideline (CPG) designed to maintain normothermia for the pediatric surgical population. This CPG outlined standard thermoregulation nursing interventions and required the consistent use of a temporal artery thermometer. A test of this CPG before full implementation established a baseline incidence of unplanned hypothermia at 16.3% (n = 80). The purpose of this study was to measure the rate of perioperative hypothermia in children after implementing the evidence-based CPG. The study results demonstrated that the CPG, guiding research-based nursing practice, consistently prevented unplanned hypothermia. The incidence rate of unplanned perioperative hypothermia after CPG implementation was 1.84% (n = 1,196).